1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a backlight unit, a liquid crystal display device having the same, and a control method thereof, and more particularly, to a backlight unit which includes a point light source, a liquid crystal display device having the same, and a control method thereof.
2. Description of the Related Art
A related art liquid crystal display device employs a light emitting diode (LED) as a light source to improve color realization, instead of a related art cold cathode fluorescent lamp (CCFL). The liquid crystal display device includes a switching element to supply a current to the LED, and controls the switching element through an analog control method or a pulse width modulation (PWM) control method.
To control the LED employing the PWM control method, an amount of the current output to the LED is controlled by determining whether a peak current flowing to the LED reaches a predetermined reference current. FIG. 1 is a graph which illustrates a waveform of an output current with respect to initial power in a related art backlight unit. An axis X in the graph represents one period of the switching element, and an axis Y in the graph refers to a waveform of the current output from the switching element. As shown therein, three current waveforms A, B and C have different switching on times Ton, respectively. The switching element is turned on until the output current reaches the reference current within one period, and turned off for a remaining time in the period.
If the period of the switching element is set as a single value, the amount of the output current increases as the switching on time Ton becomes shorter. An average current amount A′ of the current waveform A having the shortest switching on time Ton is larger than average current amounts B′ and C′ of the current waveforms B and C having longer switching on time Ton.
The switching on time Ton varies depending on a voltage level of the initial power driving the LED. If the peak output current is controlled to drive the LED, the average value of the current output to the LED changes according to the fluctuation of the initial power with respect to constant reference current and period. Here, a problem arises that brightness of the LED becomes ununiform.